This invention relates to a process and apparatus for the treatment of molten metals and, more particularly, the addition of salt flux to aluminum in melting and holding furnaces. It is claims the benefit of Provisional Application Serial No. 60/232,071, filed Sep. 2, 2000.
Treatment of molten aluminum by gases and more recently by salt fluxes in large melting and holding furnaces incorporating stirring of the molten metal has been proposed. A typical embodiment of such a device is described in the article xe2x80x9cTheoretical and Experimental Investigation of Furnace Chlorine Fluxingxe2x80x9d by Celik and Doutre in Light Metals 1989, published by the Minerals, Metals and Materials Society in 1988 (pages 793 to 800) in which an impeller positioned at an angle within the furnace is used to stir the metal in a holding furnace. A chlorine gas is added though a hole in the shaft and is entrained by the circulating metal and dispersed in the furnace. The article xe2x80x9cImproving Fluxing of Aluminum Alloysxe2x80x9d by Beland et al in Light Metals 1995, published by the Minerals, Metals and Materials Society in 1995 (pages 1189 to 1195) discloses the addition of salt flux with stirring for treatment of metal in a furnace.
Treatment of molten aluminum using salt fluxes in crucibles (for example those used for transporting molten aluminum) has been proposed. Various rotary devices have been proposed for introducing solids and/or gases into molten metal in such crucibles to perform various treatments. European Application EP 0396267, published Nov. 7, 1990 describes a system for crucible fluxing using a rotary disperser on a vertically mounted shaft into which a gas/powder mixture is fed. The disperser includes an internal structure of compartments separated by blades. It has an open bottom and as such causes metal to be pumped through and ejected from the sides of the rotor.
Another form of device for dispersing flux in a molten metal bath is described in laid open Japanese Application 1988-193136, published Jul. 28, 1988. This includes an annular rotor on a vertically mounted shaft with mixing flutes on the outer periphery thereof.
European Application EP 0395138 published Oct. 31, 1990 describes another device for dispersing materials in molten metal using a rotary system. A salt/gas mixture is injected at the underside of a generally conical injector on a vertically mounted shaft having no blades or similar shearing devices.
Canadian Application CA 2,272,976, published Nov. 27, 1999 describes a system for treatment of smelter metal in transport crucibles to reduce alkalis, by using stirrers on vertically mounted shafts. Various stirrers are disclosed, with blades mounted on the underside of a conical hollow rotor, and also including vertical blades on a portion of the upper conical surface
It is an object of the present invention to provide an improved rotary dispersing system for adding a powder/gas mixture to molten metal which is particularly well adapted for injecting a salt flux into molten aluminum in a melting or holding furnace.
According to one aspect of this invention there is provided an apparatus for the treatment of molten metal with a particulate treatment agent and a gas. This includes a vessel for holding molten metal, a rotary device for breaking up particulate treatment agent and gas within the molten metal and for dispersing particulate treatment agent and gas within the molten metal contained in the vessel and means for supplying the particulate treatment agent and gas to the rotary device. The rotary device comprises a hollow shaft having a rotor with an axial opening fixed to the discharge end of the shaft, this rotor comprising an annular plate with a plurality of radially mounted upwardly directed blades projecting from the top face of the annular plate and a plurality of radially mounted downwardly directed blades projecting from the bottom face of the annular plate.
According to a preferred feature of the invention the vessel is an aluminum melting or holding furnace and the rotary device is mounted on a carrier and can be moved in and out of an opening in the furnace. For use in this manner, the system is designed such that the rotary device can be operated with the axis of the hollow shaft at an angle of about 20-40xc2x0 to the horizontal. The carrier may be fixed adjacent to a specific opening in the furnace made for access by the rotor and shaft or it can be a mobile unit and access to the furnace is made though the normal door used for charging such a furnace.
Regardless the orientation of the rotary device within the vessel, it will be understood that the top face of the annular plate with the upwardly projecting blades is the face adjacent the hollow shaft, while the bottom face of the plate with the downwardly projecting blades is the face opposite the top face.
It is particularly preferred that the apparatus be located within the furnace so that all parts of the rotor are located at least 30 cm from any interior surface of the furnace.
Particularly when it is desired to inject a solids/gas mixture at an angle as described above, special care is required to assure that proper mixing and dispersal of the solids/gas mixture takes place.
According to a further preferred embodiment of the invention, a further annular plate is fixed below the radially mounted blades that project downwardly from the first mentioned annular plate. Thus, the rotor comprises an upper annular plate and a lower annular plate with a plurality of radially mounted, upwardly directed blades projecting from the top face of the upper annular plate and a plurality of radially mounted blades fixed between the upper and lower annular plates.
The lower annular plate preferably has a central opening communicating with the interior between the two plates.
In both of the above embodiments of the invention, the upwardly projecting radially mounted blades serve an important function. Thus, the downwardly directed blades (or the blades between the annular plates) serve to create shear and thereby to break the gas into fine bubbles and the treatment agent into fine droplets or particles. However, it has been found that there is a tendency for the bubbles generated to form high concentrations in the periphery of the rotor, that coalesce and rise rapidly to the surface, carrying the particles or droplets of treatment agent with them, thus reducing the residence time of the treatment agent in the metal. It has been found that by adding the set of upwardly directed radial blades to the rotor, a strong radial flow is generated, forcing the cloud of gas and treatment agent in a outward direction. The tendency to form high local concentrations is then reduced, increasing the average residence time of the treatment agent in the melt. These additional blades also increase the global circulation of liquid metal in the vessel.
In furnaces where the rotary device is mounted on a shaft, and the shaft is oriented at 20 to 40 degrees from the horizontal, it has been found that there is a tendency for the cloud of gas bubbles to be preferentially formed on the side of the rotor closest to the metal surface, and consequently the gas cloud, and associated treatment agent also rises more rapidly to the surface. The use of the further annular plate in accordance with one of the preferred embodiments can overcome this tendency and ensures that the cloud of gas bubbles is more uniformly dispersed around the rotor when configured in this manner.
Approximately 3-12 radially directed blades are mounted both above and below the annular plate and six blades on each location have been found to be optimum.
By being able to operate the system while mounted at an angle of about 20-40xc2x0 to the horizontal, it has the particular advantage of being capable of insertion through an opening in the side of the furnace or, if operating as a mobile unit through a loading door in the furnace. A mobile unit furthermore, can be easily transported and operated at several furnaces in the cast house.
The treatment system of this invention is well adapted for use in large commercial furnaces, e.g. furnaces having capacities of 10-150 tons. In a typical operation, a salt flux is fed at a rate of 1 kg/min in 200 l/min of carrier gas.
The method and apparatus of the invention may be used to treat a variety of molten metals with a particulate treatment agent, for example aluminum and its alloys, magnesium and its alloys, etc. The gas that is used may be inert or it may be reactive to the metal being treated. Examples of gases that may be used include chlorine, argon and nitrogen.
Examples of treatment agents which may be used in particulate form include fluxing agents such as mixtures of alkali metal chlorides for treating aluminum or its alloys, grain refiners, etc.